In modern computed tomography imaging, in order to improve sampling in the image plane (in the β direction, where β is what is termed the fan angle, see e.g. FIG. 1; the fan angle denotes an angular distance of a detector element from a central ray 12; the central ray is the centermost x-ray of the beam, connecting a focal spot 10 of a utilized x-ray source to the center of a utilized x-ray detector 11), either a so-called quarter detector offset is set or a technique known as the flying focal spot technique is used for the x-ray source. In the case of the quarter detector offset, the x-ray detector and the focal spot of the x-ray source are adjusted relative to one another such that the central ray of the focal spot is not directed precisely into the center of the detector and precisely between two detector elements, but is incident offset relative thereto by a quarter of the grid spacing of the grid of the x-ray detector (=distance between the center points of neighboring detector elements). Accordingly, the central ray also does not run through the center of rotation of the computed tomography scanner, but is offset relative thereto by a quarter of a grid spacing a of the detector elements of the x-ray detector that is projected onto the center of rotation.
The reasoning behind the quarter offset is that direct measurement rays (fan angle β, projection angle α) and measurement rays complementary to the direct measurement rays, which complementary measurement rays were received after approximately one half-revolution of the image acquisition system (x-ray source and x-ray detector) and in which the x-ray detector and the x-ray source have swapped their position (fan angle β′=−β, projection angle α′=α+π+2β), are offset relative to one another by exactly half a grid spacing. As a result, during the reconstruction into a projection, direct and complementary measurement rays can be interleaved by means of an “effective” sampling grid a/2 and consequently realize an improvement in sampling. If no quarter offset is set, the positions of the complementary measurement rays coincide with and overlay the positions of the direct measurement rays, resulting in an effective sampling grid that is equal to the grid spacing of the x-ray detector and consequently achieving no improvement in the sampling.
Alternatively, a technique referred to as flying focal spot can also be employed for improving the sampling in the image plane. Through electromagnetic deflection of the focal spot in the x-ray source (x-ray tube assembly) the position of the focal spot on an anode plate is controlled so as to produce a displacement by half a grid spacing between successive projections.
A prerequisite for the use of flying focal spot technology is an x-ray tube assembly equipped with a corresponding electromagnetic deflection means, the deflection of the focal spot having to be precisely synchronized with the readout of the x-ray detector. Setting a quarter detector offset also requires the x-ray detector and the position of the focal spot in the x-ray tube assembly to be finely adjustable. Toward that end, it is either necessary for an electromagnetic focal spot deflection device which moves the focal spot to the desired position on the anode plate to be present in the x-ray tube assembly, or else for the x-ray tube assembly and the x-ray detector to be provided with precision adjustment mechanisms. These solutions are all more or less complicated, resource-intensive and expensive. In particular, a precondition for electromagnetic focal spot deflection (for the flying focal spot or even just for setting the position of the focal spot) is an x-ray tube assembly equipped with corresponding complex and costly deflection electronics.